xscreensaver locks the screen regardless of user activity.
I type and move my mouse and my screen gets locked from xscreensaver on random. I assume that the watchdog timer, timeout timer, is not reset with every keystroke and every mouse movement! Thats the only explanation for this behaviour
qlist -Iv xscreensaver
localhost roman # qlist -Iv i3
localhost roman # qlist -Iv xorg-server
localhost roman # qlist -Iv eudev
More about in my forum post: https://forums.gentoo.org/viewtopic-t-1016950-highlight-.html
5.33 has some changes related as "Better detection of user activity on modern GNOME systems".
Do you see the same with =x11-misc/xscreensaver-5.32?